Significant amounts of water are used in the processing of various liquid streams in petroleum refineries. For example, catalytic cracking uses steam stripping for removing cracked hydrocarbons from the catalyst, aqueous amine washes are used to remove hydrogen sulfide from hydrotreating operations, water washes are used in the desalting unit and aqueous caustic washes are often used to meet final product specifications. Water may also become contaminated with hydrocarbons from rainwater run-off and in many production operations, water or steam comes into contact with hydrocarbons, for instance, in steam stimulated heavy crude production, underground cavern storage (brine contamination) and the like. Although most of the hydrocarbons can be separated from the water by simple settling procedures, the water usually remains contaminated by residual quantities of hydrocarbons. The hydrocarbon contaminants usually require removal before the water is discharged to the environment and may even require some purification before being recycled for reuse. While knock out units may be sufficient for this purpose, their effectiveness is sometimes limited.
Environmental protection regulations are usually particularly strict in terms of controlling the quantities of aromatic hydrocarbons which are allowed to enter the environment as these compounds are considered to be relatively more toxic than other hydrocarbons. Biological oxidation (biox) units capable of removing hydrocarbons are, however, prone to damage from persistent high levels of entrained hydrocarbons, high hydrocarbon concentrations resulting from unit upsets or persistent high levels of entrained aromatics which are less readily amenable to biological oxidation than paraffins. Biox plants are usually protected by an API (gravity) settler which is intended to remove undissolved hydrocarbons to low levels of a few ppm but the settlers themselves can be disabled by a number of conditions such as the presence of surfactants that create an emulsion that will not settle, high levels of aromatics which have high surface tensions and are difficult to coalesce, and slugs of hydrocarbon that are large enough to carry through the separator. There is therefore a continuing need for a reliable process for separating residual hydrocarbons from water, especially for separating aromatics from water since these present the greatest difficulty in view of their relatively greater solubility in water coupled with their grater toxicity.
A technique which has, however, become commercially attractive in recent years is liquid/liquid coalescence. See, for example, Refining Details: Advances in Liquid/Liquid Coalescing Technology, Gardner, Today's Refinery, March 1997. The method of coalescing an liquid suspended in another immiscible phase using a coalescing device frequently referred to as a coalescer, has been found useful for removing liquids both from the gaseous phase as in aerosols and from suspensions of one liquid in another liquid with which it is immiscible but may be soluble to a limited degree. Coalescing devices are particularly effective where the volume of liquid to be removed is small in comparison to the volume of the phase from which it is removed so that the technique would appear to be of potential application for the separation of small quantities of hydrocarbons from water. The Gardener article discusses the factors that are relevant to the coalescence of droplets of the discontinuous phase from the continuous phase and the ease or difficulty of separation of the immiscible phases. These factors include the physical properties of the phases such as density, viscosity, surface tension and interfacial tension. In addition, the properties of the system such as drop size, curvature of the liquid/liquid interface, temperature, concentration gradients and vibrations may also affect the effectiveness of the coalescence. As noted in U.S. Pat. No. 5,443,724 (Williamson) any or all of these factors may be significant in a particular situation but the density, drop size and interfacial tension of the two liquids appear to be the most significant factors as well as those over the least amount of control can be exercised in affecting the separations.
The separation of aromatics from water presents, as noted above, particular difficulties. Aromatics have a relatively greater water solubility than non-aromatics and may also tend to form surfactants with other materials encountered during processing. Benzene, for example, is soluble in water to the extent of about 1500 ppm whereas the solubility of iso-octane is only about 10 ppm.